Basic and translational research has investigated the role of Nm23 in the regulation of tumor metastasis. Ten transfection studies have documented that overexpression of Nm23 in various tumor cell lines resulted in a 50-90% decrease in tumor metastatic potential in vivo. The biochemical mechanism whereby Nm23 suppresses metastatic potential is under investigation. We have previously identified an interaction between Nm23-H1 and Kinase suppressor of ras, a putative scaffold protein for the Erk Map kinase complex. Nm23-H1 bound and phosphorylated serines 392 and 434 of Ksr. Transfection of breast carcinoma cells with wild type Nm23-H1 reduced Map kinase activation as compared to vector transfectant controls. We suggested the hypothesis that Nm23-H1 interaction with Ksr results in altered scaffold function, reduced Map kinase signaling and altered metastatic potential. Recently we found that Nm23-H1 expression level alters the stoichiometry of protein binding to the Ksr scaffold. In transient and stable Nm23-H1 overexpressing breast cells, greater Hsp90 is bound to Ksr than in control transfectants. The greater Hsp90 binding is accompanied by accelerated degradation of Ksr, and enhanced sensitivity to the geldanamycins in anchorage independent proliferation assays. The data indicate a potential mechanism of Nm23-H1 modulation of Ksr scaffold function, and suggest an interaction of metastatic potential and drug sensitivity. Translational research on Nm23 proposes that elevation of Nm23 expression in micrometastatic or overtly metastatic breast or other carcinomas may limit colonization, motility and de-differentiation, with a clinical benefit. Analysis of the nm23-H1 promoter revealed a 400 bp region which controlled expression, and contained a cassette of transcription factors regulated by a glucocorticoid response element (GRE). Deletion studies showed that these sites were functional in regulating nm23-H1 transcription. Medroxyprogesterone acetate (MPA), an agonist for GR, androgen receptor and progesterone receptor, elevated Nm23-H1 expression of breast carcinoma cell lines in vitro. MPA acted via a post-transcriptional mechanism using the GR, at pharmacologic doses. We have conducted preclinical experiments to determine if MPA can halt metastatic colonization. Mice were injected iv with metastatic human MDA-MB-231 breast carcinoma cells, and permitted to develop micrometastases for one month. Mice were then randomized to control vehicle or MPA, the latter given in a one month induction and subsequent bimonthly maintenance dose. Mice receiving MPA had fewer gross metastases in the lung in two experiments. Immunohistochemistry revealed that MPA treated mice had a greater proportion of pulmonary metastases with high Nm23 expression. Side effects included weight gain, but no effects on bone mineral density or mammary histology. The data indicate that agents elevating metastsis suppressor gene expression may be effective against metastatic colonization, and suggest Phase I trial of this agent for elevation of Nm23 expression as a molecular target.